Since containers formed of propylene resins are lightweight and excellent in chemical resistance and have excellent transparency, they have been widely applied also to containers for foods, toiletries, detergents, medical use, etc. As a material of conventional stretch blow molded containers, a propylene homopolymer has been exclusively used because rigidity of containers is regarded as important. As a process for industrially producing a stretch blow molded container, a process including subjecting a propylene homopolymer to melt injection molding to form a preform once, then longitudinally stretching the preform by a stretching rod and subsequently laterally stretching it by a pressurized fluid has been generally used.
However, the propylene homopolymer has a narrow stretchable temperature range, and therefore, it is necessary to strictly control the temperature in the stretching step. If this stretchable temperature range can be widened, the temperature control in summer or winter will become easy. Further, if a material having excellent stretchability can be used, a container having a thinner wall will be obtained, and the thickness of the resulting container will become more uniform, so that various performance will be improved. Accordingly, a propylene resin having stretchability superior to that of the propylene homopolymer used at present is desired.
In a patent literature 1, a polypropylene resin composition containing polypropylene having a melt flow rate (MFR), a regioirregular unit and a molecular weight distribution that are within specific ranges and a nucleating agent, and a stretch blow molded container formed of the composition are described.
In a patent literature 2, a propylene resin composition composed of a propylene/α-olefin copolymer containing an α-olefin other than propylene in an amount of 1 to 5% by weight and a propylene homopolymer or a propylene/ethylene random copolymer having an ethylene content of less than 1% by weight, and a stretch blow molded container formed of the composition are described.
However, a further new material, which has a wide moldable temperature range in stretch blow molding (such temperature range being also referred to as “stretch blow molding temperature range” hereinafter) while keeping a balance between such properties and other properties such as heat resistance and provides a container having small change in container shrinkage and transparency between before and after heat treatment, is desired as a propylene resin composition.